Accurate and Precise Determination of Silver Isotope Fractionation in

Apr 2, 2010 - High precision silver isotope ratios in environmental samples were determined by multicollector inductively coupled plasma mass spectrom...
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Anal. Chem. 2010, 82, 3922–3928

Accurate and Precise Determination of Silver Isotope Fractionation in Environmental Samples by Multicollector-ICPMS Yan Luo,†,‡ Ewa Dabek-Zlotorzynska,‡ Valbona Celo,‡ Derek C. G. Muir,§ and Lu Yang*,† Chemical Metrology, Institute for National Measurement Standards, National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6, Analysis and Air Quality Section, Air Quality Research Division, Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment Canada, Ottawa, Ontario, Canada K1A 0H3, and Aquatic Ecosystem Protection Research Division, Environment Canada, Burlington, Ontario, Canada L7R 4A6 High precision silver isotope ratios in environmental samples were determined by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Purification of Ag from sample matrixes was performed by a two stage tandem column setup with use of anion and cation exchange resin, sequentially. It was found that 1% HNO3 and 3% HCl was efficient to stabilize Ag in the final purified sample digests prior to MC-ICPMS determination. Pd at 2 µg g-1 was added to both sample and Ag standard solution as a common doping matrix as well as an internal standard for mass bias correction. Mass discrimination and instrument drift were corrected by a combination of internal normalization with Pd and standard-sample-standard bracketing, without assuming identical mass bias for Pd and Ag. NIST SRM 978a (silver isotopic standard reference material) was used for method validation and subjected to column separation and sample preparation processes. A value of -0.003 ( 0.010 ‰ for δ107/109Ag (mean and 2SD, n ) 4) was obtained, confirming accurate results can be obtained using the proposed method. To the best of our knowledge, this is the first report on δ107/109Ag variations in environmental samples. Significant differences in Ag isotope ratios were found among NIST SRM 978a standard, sediment CRM PACS-2, domestic sludge SRM 2781, industrial sludge 2782, and the fish liver CRM DOLT-4. The sediment CRM PACS-2 has a very small negative δ107/109Ag value of -0.025 ( 0.012 ‰ (2SD, n ) 4). The domestic sludge SRM 2781 has a negative δ107/109Ag value of -0.061 ( 0.010 ‰ (2SD, n ) 4), whereas industrial sludge SRM 2782 has a positive δ107/109Ag value of +0.044 ( 0.014 ‰ (2SD, n ) 4), which may indicate the contribution of Ag from different anthropogenic inputs. DOLT-4 has a much larger negative value of -0.284 ( 0.014 ‰ (2SD, n ) 4), possibly caused by biological processes. These observations confirm that Ag isotope fractionation may provide a useful tool for * Corresponding author. E-mail: [email protected]. † National Research Council Canada. ‡ Analysis and Air Quality Section, Environment Canada. § Aquatic Ecosystem Protection Research Division, Environment Canada.

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fingerprinting sources of Ag in the environment and for studying a wide variety of chemical and biological processes in nature. High precision of better than (0.015 ‰ (2SD, n ) 4) obtained in real sample matrixes makes the present method well suited for monitoring small Ag isotope fractionation in nature. Silver is a well-known environmental pollutant, and its environmental implications are of great concerns especially since new technologies, such as recent advances in nanotechnologies, have rapidly increased nanosilver (n-Ag) usage in consumer products,1 ranging from socks, paints, bandages, food containers, and even food supplements that claim to include some form of n-Ag.1,2 The increased release of n-Ag into the environment could pose potential toxic effects on aquatic organisms and may have an effect on the human life;1,2 consequently, Ag has been defined as a high priority toxic metal in risk assessment studies by Environment Canada under Chemical Management Plan (CMP) monitoring and the surveillance program list.3 On the other hand, little is known about the behavior, exposure routes, and fate of n-Ag after being released into the environment. As a result, there is an important need to develop new approaches for fingerprinting, tracking, and understanding the sources of silver in the environment. Isotope fractionation of nontraditional stable isotopes can be used as a valuable tool for studying the biogeochemical cycles of metals.4 We, therefore, propose to use high precision of Ag isotope ratio analysis by MC-ICPMS as a novel approach for fingerprinting sources of this element in the environment and for studying a wide variety of chemical and biological processes relating to Ag in nature. Notably, silver isotopic composition in the environment has not been studied in depth compared to other heavy metal pollutants such as Hg.5-19 Only recently, Yang et al.20 reported (1) Woodrow Wilson International Center for Scholars. A database of silver nanotechnology in commercial products; http://www.nanotechproject.org/ inventories/silver/; Feb. 2009. (2) Tolaymat, T. M.; El Badawy, A. M.; Genaidy, A.; Schekel, K. G.; Luxton, T. P.; Suidan, M. Sci. Total Environ. 2010, 408, 999–1006. (3) http://www.ec.gc.ca/CEPARegistry/subs_list/Domestic.cfm. (4) Johnson, C. M.; Beard, B. L.; Albare´de, F. In Geochemistry of Non-Traditional Stable Isotopes: Reviews in Mineralogy and Geochemistry; Johnson, C. M.; Beard, B. L.; Albare´de, F., Eds.; Mineralogical Society of America and the Geochemical Society: Chantilly, VA, 2004, Vol 55, pp 1-24. (5) Evans, R. D.; Hintelmann, H.; Dillon, P. J. J. Anal. At. Spectrom. 2001, 16, 1064–1069. 10.1021/ac100532r Published 2010 by the American Chemical Society Published on Web 04/02/2010

up to +0.83‰ level variations in 107Ag/109Ag ratios in several Ag fortified consumer products. Such Ag isotope variations suggested that Ag isotopic composition may be a useful tracer for studying its various chemical and biological processes in nature. However, the study of Yang et al.20 was limited to a few Ag fortified consumer products using MC-ICPMS detection. Therefore, applications of high precision Ag isotope determination in real environmental samples are urgently required. Previous studies with use of MC-ICPMS have reported precisions of Ag isotope ratios in geological samples in a range of 0.13-2‰21,22 to 0.05‰ by Schonbachler et al.23,24 The study of Yang et al. also achieved a similar precision of 0.05‰ for Ag fortified consumer products.20 However, such precision might not be enough to distinguish small fractionation of Ag isotopes in the environment. The purpose of this work was to develop a more precise and optimized method for the accurate and precise determination of Ag isotope ratios in a few typical environmental sample matrixes including a biological tissue, sediment, and sludge to prove the concept. In this study, mass discrimination and instrument drift are corrected for by implementation of a combination of internal normalization with Pd and standardsample-standard bracketing, without assuming identical mass bias for Pd and Ag, similar to a previous used method.20 EXPERIMENTAL SECTION Instrumentation. A Thermo Fisher Scientific Neptune (Bremen, Germany) MC-ICPMS equipped with 9 Faraday cups and a combination of cyclonic and Scott-type spray chambers with a PFA self-aspirating nebulizer MCN50 (Elemental Scientific, Omaha, NE) operating at 50 µL min-1 was used for all measurements. The plug-in quartz torch with sapphire injector was fitted with a platinum guard electrode. Low resolution mode was used in (6) Lauretta, D. S.; Klaue, B; Blum, J. D.; Buseck, P. R. Geochim. Cosmochim. Acta 2001, 65, 2807–2818. (7) Hintelmann, H.; Lu, S. Analyst 2003, 128, 635–639. (8) Jackson, T. A.; Muir, D.; Vincent, W. F. Environ. Sci. Technol. 2004, 38, 2813–2821. (9) Smith, C. N.; Kesler, S. E.; Klaue, B.; Blum, J. D. Geology 2005, 33, 825– 828. (10) Xie, Q.; Lie, S.; Evans, D.; Dillon, P.; Hintelmann, H. J. Anal. At. Spectrom. 2005, 20, 515–522. (11) Ridley, W. I.; Stetson, S. J. Appl. Geochem. 2006, 21, 1889–1899. (12) Kritee, K.; Blum, J. D.; Johnson, M. W.; Bergquist, B. A.; Barkay, T. Environ. Sci. Technol. 2007, 41, 1889–1895. (13) Blum, J. D.; Bergquist, B. A. Anal. Bioanal. Chem. 2007, 388, 353–359. (14) Zheng, W.; Foucher, D.; Hintelmann, H. J. Anal. At. Spectrom. 2007, 22, 1097–1104. (15) Yang, L.; Sturgeon, R. E. Anal. Bioanal. Chem. 2009, 393, 377–385. (16) Malinovsky, D.; Sturgeon, R. E.; Yang, L. Anal. Chem. 2008, 80, 2548– 2555. (17) Bergquist, B. A.; Blum, J. D. Science 2007, 318, 417–420. (18) Jackson, T. A.; Whittle, D. M.; Evans, M. S.; Muir, D. C. G. Appl. Geochem. 2008, 23, 547–571. (19) Muir, D. C. G.; Wang, X.; Yang, F.; Nguyen, N.; Jackson, T. A.; Evans, M. S.; Douglas, M.; Ko¨ck, G.; Lamoureux, S.; Pienitz, R.; Smol, J. P.; Vincent, W. F.; Dastoor, A. Environ. Sci. Technol. 2009, 43, 4802–4809. (20) Yang, L.; Dabek-Zlotorzynska, E.; Celo, V. J. Anal. At. Spectrom. 2009, 24, 1564–1569. (21) Carlson, R. W.; Hauri, E. H. Geochim. Cosmochim. Acta 2001, 65, 1839– 1848. (22) Woodland, S. J.; Rehka¨mper, M.; Halliday, A. N.; Lee, D. C.; Hattendorf, B.; Gu ¨ nther, D. Geochim. Cosmochim. Acta 2005, 69, 2153–2163. (23) Scho ¨nba¨chler, M.; Carlson, R. W.; Horan, M. F.; Mock, T. D.; Hauri, E. H. Int. J. Mass Spectrosc. 2007, 261, 183–191. (24) Scho ¨nba¨chler, M.; Carlson, R. W.; Horan, M. F.; Mock, T. D.; Hauri, E. H. Geochim. Cosmochim. Acta 2008, 72, 5330–5341.

Table 1. MC-ICPMS Operating Conditions instrument settings Rf power plasma Ar gas flow rate auxiliary Ar gas flow rate Ar carrier gas flow rate sampler cone (nickel) skimmer cone (H, nickel) lens settings data acquisition parameters scan type cup configuration resolution sensitivity blank 1% (v/v) HNO3 and 3% HCl integration time number of integrations cycles/blocks

1250 W 15.0 L min-1 1.00 L min-1 1.087 L min-1 1.1 mm 0.8 mm optimized for maximum analyte signal intensity static measurements (104Pd)L3, (105Pd)L2, (107Ag)C, and (109Ag)H2 ∼300 43 V/ppm for 107Ag 0.0016 V for 107Ag, 0.0015 V for 109Ag, 0.005 V for 104Pd, and 0.011 V for 105Pd 4.194 s 1 10/5

this study with Rpower(5, 95%) ∼ 300. Optimization of the Neptune was performed as recommended by the manufacturer; typical operating conditions are summarized in Table 1. A quadrupole ICPMS DRC ELAN6100 (PerkinElmer Sciex, Thornhill, Ontario, Canada) or a sector field ICPMS Element2 from Thermo Fisher Scientific (Bremen, Germany) was used for a semiquantitative analysis. A Digiprep Jr block heater and 50 mL volume Teflon tubes (SCP Science, Quebec, Canada) were used for evaporation of samples. A Thermo IEC Centra CL3 (Thermo Fisher Scientific, North Carolina) was used for centrifuging samples. Empty 2 mL columns (part number: AC-141-AL) and 25 mL extension funnels (part number: AC-120) were purchased from Eichrom Technologies LLC (Darien IL, USA). Reagents and Solutions. Reagent grade nitric and hydrochloric acids (Fischer Scientific Canada, Ottawa, Ontario, Canada) were purified in-house prior to use by sub-boiling distillation of reagent grade feedstock in a quartz still. Environmental grade HF and H2O2 was purchased from Anachemia Science (Montreal, Quebec, Canada). High-purity (18 MΩ cm) deionized water (DIW) was obtained from a NanoPure mixed bed ion exchange system fed with reverse osmosis domestic feedwater (Barnstead/Thermolyne Corp, Iowa). SRM 978a (Ag isotope) in AgNO3 form was obtained from National Institute of Standards and Technology (NIST, Gaithersburg, MD). A 1000 µg g-1 stock solution of SRM 978a Ag was prepared by its quantitative dissolution in 2% (v/v) HNO3 solution. A 3000 µg g-1 stock solution of Pd was prepared by quantitative dissolution of high purity Pd metal (Johnson Matthey Plc) in a few milliliters of HNO3 and diluted with DIW. A 800 µg g-1 Pd working standard solution was prepared by a quantitatively dilute suitable amount of stock solution in 2% (v/v) HNO3. AG1-X8 anion exchange resin (100-200 mesh) and AG50W-X8 cation exchange resin (200-400 mesh) were obtained from Eichrom Technologies LLC (Darien, IL). National Research Council Canada (NRCC, Ottawa, Canada) dog fish liver CRM DOLT-4 and harbor sediment CRM PACS-2 were used for Ag isotope ratio determination. Industrial sludge SRM 2782 and domestic sludge SRM 2781 were purchased from National Institute of Standards and Technology (Gaithersburg, MD). Analytical Chemistry, Vol. 82, No. 9, May 1, 2010

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Table 2. Column Separation Procedure

a

anion exchange column AG1-X8

step

3 × 10 mL 2 M HCl 3 × 5 mL 0.5 M HCl sample in 50 mL 0.5 M HCl 5 × 10 mL 0.5 M HCl 3 × 10 mL 0.1 M HCl 3 × 5 mL 0.001 M HCl 9 × 5 mL 1 M HNO3

clean condition load rinse rinse rinse elutea

cation exchange column AG50W-X8

step

3 × 10 mL 6 M HNO3 3 × 10 mL 1 M HNO3 9 × 5 mL 1 M HNO3

clean condition load + elutea

Tandem column setup.

Sample Digestion. Sample preparation was undertaken in a class-100 clean room. For sediment CRM PACS-2, 0.25 g subsamples were weighed into individual precleaned Teflon digestion vessels. Eight milliliters of HNO3, 1 mL of HF, and 0.5 mL H2O2 were added to each vessel. Two procedure blanks were also prepared concurrently. The sealed vessels were heated in a Milestone microwave oven operated under the following conditions: 10 min at 50 °C and 500 W; 10 min at 140 °C and 500 W; 15 min at 180 °C and 600 W; and 45 min at 210 °C and 650 W. After cooling, the caps were removed and 6 mL of 5 M HCl were added to each vessel. The vessels were placed on a hot block and heated at 120 °C for 1 h to avoid any AgCl precipitates in the solution under excess amount of Clconcentration. Then, the contents were transferred into 50 mL Teflon tubes and placed on a hot block in a class-10 fume hood and heated at 120 °C until dryness. Following addition of 1 mL HCl, the contents were heated at 105 °C until dryness. The residues were redissolved in 5 mL of 5 M HCl with heating for 30 min at 85 °C followed by dilution with DIW to 50 mL which resulted in a final concentration of 0.5 M HCl. The solutions were sonicated for 30 min to ensure complete dissolution followed by centrifugation for a few minutes before loading the supernatant onto the column for matrix separation. Fish liver CRM DOLT-4 and two sludge SRMs were prepared similarly as described above with the exception of HF was not used for DOLT-4. Since both sludge SRM samples contain high concentration of Ag, 0.050 g of subsamples instead of 0.25 g were used. Column Separation. Anion exchange columns containing 1.25 mL each of AG1-X8 resin and cation exchange columns containing 1.80 mL each of AG50W-X8 resin were cleaned and preconditioned with adequate acids as detailed in Table 2. After the sample loading onto the anion columns in 0.5 M HCl, the columns were rinsed each with 5 × 10 mL of 0.5 M HCl and 3 × 10 mL of 0.1 M HCl, followed by 3 × 5 mL of 0.001 M HCl. The rack holding anion exchange columns were then placed on the top of the other rack holding cation exchange columns (Figure 1). To elute anion exchange columns and at same time to load and elute cation exchange columns, 9 × 5 mL of 1 M HNO3 were passed through each set of tandem columns and collected into 50 mL Teflon tubes. The collected fractions were dried on a hot plate at 120 °C to dryness. The whole tandem column separation procedure was repeated one more time. The final collected total 45 mL 3924

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Figure 1. Tandem column setup.

of 1 M HNO3 fractions were dried on a hot plate at 120 °C to dryness. The residues were redissolved in a mixture of 0.05 mL of HNO3, 0.15 mL of HCl, and 0.3 mL of DIW at 105 °C for 30 min followed by dilution to 5 mL with DIW prior to MCICPMS measurements. An aliquot of 0.50 mL each of these purified solutions was taken out for total semiquantitative analysis to check left over matrix elements. The remaining purified solutions were each spiked with 0.0113 g of 800 µg g-1 Pd standard solution prior to MC-ICPMS ratio determination. Analysis Procedure and Data Processing. Samples and standards were introduced into the plasma using a self-aspiration nebulizer at 50 µL min-1. Intensities of Ag and Pd isotopes obtained (shown in Table 1) from a procedure blank solution in 1% (v/v) HNO3 and 3% (v/v) HCl were subtracted from all samples and standards. Samples were introduced in the sequence of SRM 978a-sample-SRM 978a. A static run was employed to collect 104Pd, 105Pd, 107Ag, and 109Ag isotopes on Faraday cups at L3, L2, Central, and H2 positions, respectively. Data acquisition parameters are summarized in Table 1. Five measurements were made on each test sample solution. A bracketing standard solution of SRM 978a in 1% (v/v) HNO3 and 3% (v/v) HCl was prepared for each sample so as to contain similar concentrations of Ag and Pd. Major matrix elements and elements which could pose potential polyatomic interferences on Ag and Pd isotopes were assessed in the samples using a quadrupole ICPMS or a sector field ICPMS and semiquantitative analysis mode. RESULTS AND DISCUSSION Optimization and Mass Bias Correction. Good measurement precision for Ag isotope ratio was achieved using an integration time of 4.194 s acquiring data with 10 cycles and 5 blocks within a reasonable measurement time. One measurement

where Rsample and Rstd are mass bias corrected (or “true”) ratios T T in the sample and standard, respectively. On the basis of this approach, a δ107/109Ag of -0.002 ± 0.009‰ (mean and 2SD, n ) 5) in a 100 ng g-1 solution of SRM978a containing 2 µg g-1 Pd was obtained, confirming accurate and precise Ag ratio determination using the present mass bias correction method. Tandem Column Matrix Separation. In addition to instrumental mass bias, sample matrix could have large effects on accurate ratio measurements. To properly correct for these effects, separation of analyte from sample matrix is generally necessary,20 since in practice it is impossible to match all matrix elements between a sample and a standard solution. It is now widely recognized that nonquantitative recovery of analyte as a result of

matrix separation may induce isotope fractionation27,28 and quantitative (above 95%) recovery of analyte is, thus, required to ensure accurate results. During our preliminary studies, an Ag standard solution at 500 ng g-1 was used to test recoveries of anion and cation exchange columns, respectively. It was found that eluting the anion exchange column with 1 M HNO3 instead of 0.5 M HNO3 used previously21-24 significantly decreased the total volume of the acid solution needed to quantitatively elute Ag from this column. In addition, quantitative recovery of Ag from the cation exchange column was obtained when 1 M HNO3 was used for loading and elution. Thus, 1 M HNO3 was used for all subsequent experiments. To improve the sample throughput of column separation and sample preparation, a tandem column setup was applied (Figure 1), after sample loading and rinsing of the anion exchange column separately (see Table 2). This way, the eluent from the anion exchange column was directly dripped into the cation exchange column for loading and eluting simultaneously. On the basis of a 0.25 g subsample for a PACS-2 digest, a 25 mL aliquot of 1 M HNO3 is enough to quantitatively elute Ag from the tandem column setup. However, for samples containing higher concentration of Ag such as SRM 2781 (98 ± 8 µg g-1), a 45 mL aliquot of 1 M HNO3 is needed for complete elution of Ag from the columns. Thus, a 45 mL aliquot of 1 M HNO3 was used in all subsequent experiments. The present tandem column separation setup allows a batch separation (usually 12 samples at a time) to be finished within 2 days, an improvement compared to 3 days required in previous studies.23,24 Initially, the collected 1 M HNO3 eluent (45 mL) was used for Ag ratio determination and for TotalQuant measurements to check the efficiency of matrix separation using tandem column setup. However, a loss of Ag signal in a collected 45 mL, 1 M HNO3 was observed as this solution aged and the Ag signal completely disappeared after the solution was left to stand for 2 weeks. This is likely due to a trace amount of Clpresent in this solution from the 0.001 M HCl used for rinsing of an anion exchange column and, thus, precipitates Ag out from the solution with time. To overcome this, the collected Ag fraction in 45 mL of 1 M HNO3 was heated to dryness at 120 °C. The residues were then redissolved in a mixture of 0.05 mL of HNO3, 0.15 mL of HCl, and 0.3 mL of DIW at 85 °C for 30 min, followed by dilution to 5 mL with DIW resulting in a 1% (v/v) HNO3 and 3% (v/v) HCl solution. Under these conditions, the Ag concentration was found to be very stable. Spike recovery tests were conducted on PACS-2 digests with a 2 µg Ag spike, and quantitative recovery of 99.8 ± 2% (2SD, n ) 4) Ag was obtained. Washing the columns prior to use ensured that the overall column separation procedural blank was less than 0.3 ng Ag (absolute), about 200-fold less than the absolute amount of Ag in the processed subsamples. However, intensities for Ag and Pd isotopes in all samples were procedure blank corrected in order to achieve the best results. Processing a 100 ng g-1 NIST 987a Ag standard through the columns did not induce any detectable mass fractionation of Ag ratio in the purified solutions, (δ107/109Ag value of -0.001 ± 0.010‰, mean and 2SD,

(25) Yang, L. Mass Spectrom. Rev. 2009, 28, 990–1011. (26) Russell, W. A.; Papantastassiou, D. A.; Tombrello, T. A. Geochim. Cosmochim. Acta 1978, 42, 1075–1090.

(27) Wieser, M. E.; De Laeter, J. R.; Varner, M. D. Int. J. Mass Spectrom. 2007, 265, 40–48. (28) Irisawa, K.; Hirata, T. J. Anal. At. Spectrom. 2006, 21, 1387–1395.

requires about 7 min, including uptake and washout, consuming approximately 0.35 mL of sample. In this study, a high performance skimmer cone for Neptune was used to improve sensitivity for high precision Ag isotope ratio measurements in generally low Ag containing environmental samples. As detailed in a recent review,25 for accurate and precise isotope ratio determination, it is essential to correct for mass discrimination observed in MC-ICPMS. It is a common practice to use internal standard for mass bias correction of analyte isotope ratios. However, the assumption of identical mass bias correction factors for both analyte (e.g., Ag) and internal standard (e.g., Pd), used in previous studies,21-24 is both unnecessary and incorrect.20,25 Thus, a mass bias correction approach based on internal normalization with Pd in combination with standard-sample-standard bracketing used previously for Ag isotope ratio determination in commercial products20 was adopted here. However, the isotope pair of 104Pd/105Pd instead of 106Pd/108Pd, which was used previously,20 was chosen in this study to avoid any possible 106 Cd interference on 106Pd and 108Cd interference on 108Pd due to a small amount of Cd left over in the purified sample digests after column separation. The “certified” value of 1.07638 for 107 Ag/109Ag in SRM 978a was used to obtain mass bias corrected 104Pd/105Pd ratios in two adjacent standard solutions of SRM 978a, and their average value was then used to calculate mass bias corrected Ag isotope ratios in the samples based on an exponential law for mass bias correction26 used in the software of Neptune:

Rm ) RT ·

() mi mj

f

(1)

where subscripts m and T indicate measured and mass bias corrected ratios, respectively; f is the mass bias correction factor; mi and mj are the absolute masses of the isotopes of interest. For comparison purposes, a δ-value scale established relative to a common standard solution is used for this study.

δ107/109Ag )

(

Rsample T Rstd T

)

- 1 · 1000‰

(2)

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Table 3. Semiquantitative Results for Matrix Elements in Purified Sample Digests, Ratio of Individual Matrix Element Concentration to Ag Concentration in the Digestsa analyte

DOLT-4 (46.5 ng g-1 PACS-2 (60 ng g-1 SRM 2781 (490 ng g-1 SRM 2782 (153 ng g-1 Ag in purified sample solution) Ag in purified sample solution) Ag in purified sample solution) Ag in purified sample solution)

# of column separation

once

twice

once

twice

once

twice

once

twice

Na Mg Al K Ca Ti Zn Cd Sb Sn Ba Bi Nb W Tl Pb U

0.08 0.03 0.05 0.09